mounted in a vacuum vessel and conduction-cooled with. Gifford-McMahon cycle cryocoolers is used to develop and refine design and construction techniques.
BNL-95153-2011-CP
Design, Construction and Test of Cryogen-Free HTS Coil Structure H. Hocker, M. Anerella, R. Gupta, S. Plate, W. Sampson, J. Schmalzle, Y. Shiroyanagi Brookhaven National Laboratory, Upton, NY 11973 U.S.A.
Presented at the Particle Accelerator Conference (PAC11)
New York City, NY March 28 – April 1, 2011
May 2011
Superconducting Magnet Division
Brookhaven National Laboratory
U.S. Department of Energy University of Michigan Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. This preprint is intended for publication in a journal or proceedings. Since changes may be made before publication, it may not be cited or reproduced without the author’s permission.
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
BNL-95153-2011-CP Submitted at the Particle Accelerator Conference (PAC11), New York City, NY - March 28-April 1, 2011
Design, Construction and Test of Cryogen-Free HTS Coil Structure* H. Hocker, M. Anerella, R. Gupta, S. Plate, W. Sampson, J. Schmalzle, Y. Shiroyanagi, BNL, Upton, New York 11973, U.S.A. Abstract This paper will describe design, construction and test results of a cryo-mechanical structure to study coils made with the second generation High Temperature Superconductor (HTS) for the Facility for Rare Isotope Beams (FRIB). A magnet comprised of HTS coils mounted in a vacuum vessel and conduction-cooled with Gifford-McMahon cycle cryocoolers is used to develop and refine design and construction techniques. The study of these techniques and their effect on operations provides a better understanding of the use of cryogen free magnets in future accelerator projects. A cryogen-free, superconducting HTS magnet possesses certain operational advantages over cryogenically cooled, low temperature superconducting magnets.
surrounding aluminum, and the enclosing cryostat structure contain a rectangular inner opening allowing for addition of warm iron in the future.
CONSTRUCTION DETAILS Six coils have been constructed with the second generation (2G) HTS based on essentially the same geometry that was earlier used in making twenty five coils with the first generation HTS [1] for Facility for Rare Isotope Beams [2]. Four of these coils are made with 100 meter of ~4 mm wide and ~0.3 mm thick HTS tape from American Superconductor Corporation (ASC) and two with 100 meter of ~4 mm wide and ~0.1 mm thick tape from SuperPower. SuperPower coils (smaller in width) are put in middle of the six coil pack.
Figure 1: Completed Magnet The six-coil pack is constrained by an assembly of ten aluminum plates which align the coils and provide the desired preload to the coil stack. See Figure 2. This coil box (cold mass) is suspended from the stainless steel cryostat lid using G-10 links to minimize heat leak (See Figure 3 & Figure 4). Design of the coil pack, _____________________ *This work is supported by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886 and under Cooperative Agreement DE-SC0000661 from DOE-SC that provides financial assistance to MSU to design and establish FRIB.
Figure 2: Cold Mass Box with 6-Coil Pack The lid has welded attachments allowing for connection of the cryocoolers, leads, instrumentation, pump-out port, and pressure relief. The cryostat volume is completed by attachment of the lid assembly to an aluminium cryostat box, machined from solid aluminium billet without welding. Attachment of the lid to the cryostat box is accomplished with fasteners and an O-Ring seal. This method allows for simple disassembly for inspection or changes. The cold mass is connected thermally to the first of two Gifford-McMahon (G-M) Cryocoolers [3] using two laminated straps fabricated from .010” (0.25 mm) thick copper strips, attached to opposite ends of the cold mass. Using laminated sheets instead of a solid bus provides sufficient flexibility to account for thermal shrinkage. Coil electrical leads and external power leads all terminate at a copper power lead head (See Figure 4). The lead head is connected to the second G-M cryocooler. Thin Kapton sheets electrically isolate the cryocooler head from the energized copper lead terminations. Leads travelling to the outside of the magnet have been lengthoptimized to minimize both heat leak and internal heat generation in those non-superconducting regions and are cooled only by conduction from the cryocooler. The system is designed to test coils up to a current of 500A. By assuming a set of end temperatures for the magnet leads, the length-optimization calculation provided the lead length which would offer the minimum heat generation with those end temperatures. Assuming lead temperatures of 293K & 80K, the optimization anticipated a load of 29W for each lead @ operating
BNL-95153-2011-CP Submitted at the Particle Accelerator Conference (PAC11), New York City, NY - March 28-April 1, 2011
current. Using separate cryocoolers for the leads and the coil box allow for operation of the lead cryocooler at higher temperature than that required for coil operations. In this way, the design could take advantage of the cryocoolers increased capacity in that range. Since cooling was entirely dependent on conduction, joints were considered critical. Apiezon-N vacuum grease [4] was used as an interposer in the bolted connections to increase thermal conductivity. At these connections, maximum torque was used in conjunction with Belleville washers, to maintain contact pressure at operational temperatures. Temperature instrumentation was provided by silicon diodes placed at thirty two locations within the magnet structure. As thermal behaviour of the joints was of particular interest, diodes were placed at nearly all of the junctions in the heat-path. Additional diodes were located on the coils, the cryocooler ends, the leads, and one of the G-10 straps holding up the magnet. Redundant sensors were placed at a number of locations. This was done to study consistency in readings and to confirm the effectiveness of the various attachment methods used for locating the diodes.
OPERATIONAL RESULTS Initial operations involved cooling without application of current. After pump-down and cryocooler activation, steady state was reached in approximately 20 hours (see Figure 5). Select steady state temperatures are given in Table 1. Select temperature gradients at heat-carrying mechanical connections are given in Table 2. Table 3 shows actual vs. analysis generated temperatures for selected points in the magnet. Table 4 shows values of conductance used in obtaining the values in Table 3. Table 1: Select Steady State Temperatures Location
Temperature (K)
Lead 1 - Warm End /Cold End
248 /42
Lead 2 - Warm End /Cold End
247 /42
Cryocooler Head (Coil)
26
Cryocooler Head (Leads)
33
Coil (Average)
39
Coil Box (Range)
29 - 38
Table 2: Select ∆T for Mechanical Connections Location
∆T (K)Range
Cryocooler Head – Copper Block
0-1.3
Copper - Copper (2 Mil Kapton Interposer)
7.7-8.7
Copper - Copper (Apiezon-N Interposer)
